The present invention relates to a rotary position detector for a stepping motor, and more particularly to a rotary position detector for a stepping motor whose rotary position is detected by counting driving pulses for driving the stepping motor or fractions thereof.
As is well known in the art, a stepping motor makes an equi-angular rotation on each reception of a driving pulse. The rotary position or angle of the stepping motor can in theory be precisely obtained by calculation. That is, the amount of rotation is calculated by multiplying the rotary angle per driving pulse by the number of supplied driving pulses which is counted by a counter. However, if a rotary discrepancy called step-out occurs, the actual amount of rotation differs from the amount of rotation measured in the counter.
To eliminate the influence of the step-out of the stepping motor, the counter for counting the number of driving pulses is reset with a reset pulse which is generated every one revolution of the stepping motor. Accordingly, even if there occurs a step-out in one revolution, the actual rotary position in the next revolution can be precisely obtained after the counter is reset.
For obtaining such reset pulse, a signal means is provided on the rotary shaft of the stepping motor or on a rotary member secured to the rotary shaft to produce a reset signal each revolution of the stepping motor, as disclosed in, for instance, U.S. Pat. No. 4,687,981.
In U.S. Pat. No. 4,687,981, there is provided a radially extending light-shielding lug as a signal means on the rotary shaft of a stepping motor or a disc on the rotary shaft with a hole therethrough. When the light-shielding lug or the hole is detected with a photosensor, the photosensor produces a reset signal each revolution of the rotary shaft. The structure for provision of the signal means is simple. However, if the step-out occurs, precise counting of driving pulses and the determination of the rotated position of the stepping motor must be delayed until the counter is reset by a reset signal after, at most, one revolution of the rotary shaft.
In U.S. Pat. No. 4,534,560, four light-shielding plates are provided as signal means, which differ in shape from each other, so as to shorten the time until the counter is reset. The four light-shielding plates are secured to a rotary reel on the motor shaft to define four regions into which the reel is divided. A photosensor produces four kinds of reset signals for resetting the counter. The precise counting of driving pulses and determination of the rotary position of the stepping motor can be done each quarter of a revolution of the stepping motor, utilizing the reset signals. However, the signal means are complicated in shape, and circuitry is necessary for distinguishing the four kinds of signals corresponding to the four signal means.